Updated: Sep 27, 2020
If you have a newer GPS watch, it likely provides all sorts of metrics tracking your “fitness,” “Vo2Max,” “economy,” or similar. While these figures can sometimes show a correlation with your actual fitness when tracked over time, they’re not truly reliable. I recommend that my athletes ignore them.
These numbers generally track the relationship between heart rate and running pace. Sometimes they take into account HRV, and occasionally even the altitude and elevation gain on your run.
So with all that data, why are they inaccurate? First, the source data is often faulty. Wrist-based HR is notoriously unreliable, especially in the initial few miles of a run. Running pace calculation is dependent on satellite reception and can vary significantly depending on terrain, tree cover, and even cloud cover and what you’re wearing. Even the elevation gain isn’t always accurate. Combining multiple inaccurate data points just compounds to make more inaccurate data.
Second, and more importantly, a change in these metrics day to day is totally unimportant. It’s totally normal that your heart rate, pace, perceived effort, muscle soreness and how they all relate to each other will change from day to day. This isn’t any sign of your fitness, and, with a very few exceptions, should not be used to make training decisions on a day to day basis.
Finally, calling it VO2Max is very misleading. You cannot measure VO2max via heart rate alone; it’s literally a measure of oxygen consumption. Additionally, VO2Max isn’t as trainable as running economy. You may see some improvements with training, but the watch can’t measure them.
So, what data SHOULD you focus on? My simple answer is, focus on the fewest metrics that are truly helping you make meaningful training decisions. My long answer? Check out my next post!
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